Industrial-grade mixing devices are generally divided into classes based upon their ability to mix fluids. Mixing is the process of reducing the size of particles or inhomogeneous species within the fluid. One metric for the degree or thoroughness of mixing is the energy density per unit volume that the mixing device generates to disrupt the fluid particles. The classes are distinguished based on delivered energy densities. There are three classes of industrial mixers having sufficient energy density to consistently produce mixtures or emulsions with particle sizes in the range of 0 to 50 microns.
Homogenization valve systems are typically classified as high energy devices. Fluid to be processed is pumped under very high pressure through a narrow-gap valve into a lower pressure environment. The pressure gradients across the valve and the resulting turbulence and cavitation act to break-up any particles in the fluid. These valve systems are most commonly used in milk homogenization and can yield average particle sizes in the 0-1 micron range.
At the other end of the spectrum are high shear mixer systems, classified as low energy devices. These systems usually have paddles or fluid rotors that turn at high speed in a reservoir of fluid to be processed, which in many of the more common applications is a food product. These systems are usually used when average particle sizes of greater than 20 microns are acceptable in the processed fluid.
Between high shear mixer and homogenization valve systems, in terms of the mixing energy density delivered to the fluid, are colloid mills, which are classified as intermediate energy devices. The typical colloid mill configuration includes a conical or disk rotor that is separated from a complementary, liquid-cooled stator by a closely-controlled rotor-stator gap, which is commonly between 0.001-0.40 inches. As the rotor rotates at high rates, it pumps fluid between the outer surface of the rotor and the inner surface of the stator, and shear forces generated in the gap process the fluid. Many colloid mills with proper adjustment achieve average particle sizes of 1-25 microns in the processed fluid. These capabilities render colloid mills appropriate for a variety of applications including colloid and oil/water-based emulsion processing such as that required for cosmetics, mayonnaise, or silicone/silver amalgam formation, to roofing-tar mixing.
Existing colloid mills have suffered from a number of performance- and ease-of-use-related problems.
One such problem relates mechanical complexity and stability. In the past, colloid mills have had mill housings for the rotor/stator and separate electrical motors with direct drive, reduction gear-, or belt-drive systems connecting the motors to the mill rotors. Elaborate mechanical isolation is required because both the mill rotor and the electric motor have separate bearing systems. Furthermore, the mechanisms used to enable rotor-stator gap adjustment, worm gear arrangement in one commercial device, have been mechanically complex and potentially dynamic during operation primarily due to thermal expansion effects.
In the present invention, these problems are avoided by relying on a motor-driven shaft configuration. That is, the shaft that drives and connects to the rotor of the colloid mill extends to the electric motor stator of the electric motor. In this way, the mill rotor shaft is directly driven.
The benefits resulting from this configuration primarily concern simplicity. Complex gear or belt drive arrangements between a separate electric motor and the fluid processing components of the colloid mill are avoided. Moreover, the gap between the mill rotor and mill stator can be adjusted simply by axially translating the motor-driven shaft. The small movements, of typically less than a 0.1 inches, have no or negligible effect on the electromagnetic field generation in the electric motor. Moreover, in this configuration, only one set of thrust bearings are required, and these are located very close to the rotor, thus minimizing any thermal expansion effects on the mill rotor-stator gap.
In general, according to one aspect, the invention features a colloid mill comprising a mill stator, a mill rotor, an electric motor stator, and a motor-driven shaft. This motor-driven shaft functions as an electric motor rotor that operates in cooperation with the electric motor stator, but also extends from the electric motor stator to the mill rotor, providing a direct drive arrangement.
In specific embodiments, a gap adjustment system is provided that changes a gap between the mill stator and the mill rotor by axially translating the motor-driven shaft relative to the electric motor stator. Further, the electric motor driven shaft is axially supported to counteract forces generated between the mill stator and mill rotor by at least one thrust bearing, preferably an angular contact bearing set, that is located on the side of the electric motor stator proximal to the mill rotor. As a result, mere radial support bearings are needed on the distal side of the electric motor stator relative to the mill rotor.
Another problem that arises in existing colloid mill designs is related to the stability of the mill rotor-stator gap and specifically the system used to adjust the gap. One of the most common configurations utilizes a worm-gear arrangement. This system, however, is hard to calibrate and can jam or freeze in response to the forces generated between the mill rotor and stator.
This problem is solved in the present invention by providing a timing belt-based arrangement for adjusting the gap. Such a timing belt system provides for no backlash. As a result, a simple hand-operated knob or stepper motor arrangement can be used to control the gap.
Specifically, a thrust bearing is supported in a threaded sleeve that mates with the colloidal mill body. The timing belt engages the sleeve to rotate it relative to the body, thus adjusting the thrust bearings axially and thereby controlling the gap between the mill stator and mill rotor.
In general, according to another aspect, the invention features a gap adjustment system for a colloid mill. The system comprises at least one thrust bearing that supports a shaft carrying a mill rotor in proximity to a mill stator. A threaded sleeve in turn carries the thrust bearing, its threads mating with complimentary threads of a body of the colloid mill. A timing belt, which is supported by the colloid mill body, engages the threaded sleeve to enable rotation relative to the body to thereby translate the thrust bearings, yielding axial movement of the shaft. This changes the gap between the mill stator and mill rotor.
In specific embodiments, a knob is used to manually adjust the timing belt.
In other embodiments, an adjustment motor, such as a stepper motor is used to adjust the timing belt under microprocessor control.
Another problem that arises in existing mills concerns what happens when a customer requires a new colloid mill for a given manufacturing process to handle higher fluid processing rates. In the past, manufacturers have offered larger and smaller-sized colloid mills to meet customer demand. The problem, however, has been that typically when moving to colloid mills of a higher throughput the manufactures have simply offered larger versions of a geometrically similar mill rotor-stator configuration. Put another way, a colloid mill with a higher throughput had a rotor and stator that looked like the colloid mill with a lower throughput but were simply larger. This technique for modifying colloid mill rotor/mill stator configurations to handle higher fluid volumes yields different processing effects on those fluids. The larger colloid mills tended to process the fluid at different energy densities, typically higher than the smaller colloid mills. This was a problem to the customer since it required recalibration of the processing parameters of the fluid in order to maintain a consistent product.
The present invention uses the recognition that the energy density delivered to the fluid or the characteristics that provide a uniform particle size at the output is related to the third power of the rotor speed and the second power of the rotor diameter. As a result, when scaling mill rotor/mill stator configurations to higher fluid throughput and consequently larger rotors, it is necessary to decrease the rotor speed. In order that the fluid has a consistent residence time and velocity gradient in the mill rotor-stator gap, the surface angle or rotor pitch, however, is increased with increases in the size of the rotor to counteract the effects of the slower rotor speeds. This provides kinematic similarity, or similar changes in velocity as the product traverses the mill rotor-stator gap of different sizes of the colloid mill.
In general, according to another aspect, the invention features a family of colloid mills in which the rotor surface pitch angles increase with increases in colloid mill throughputs. Said another way, the mill rotor surface angles and rotor surface lengths are controlled between colloid mills having different throughput in order to standardize the energy input into the processed fluids.
Another problem with existing mills has been colloid mill rotor configurations. Some mills have long slots that extend down the entire face of the mill rotor, whereas other configurations utilize relatively smooth conical- or disk-shaped rotor configurations. Each configuration has its relative advantages and disadvantages. The smooth rotor configuration tends to generate high and consistent shear forces in the processed fluid. The configuration with the long axially and radially running slots provides high fluid throughput rates, while establishing good turbulence.
The present invention utilizes a largely smooth rotor configuration in order to generate uniformly high shear forces, and thus consistency with correspondingly low variance in the particle size in the processed fluid. The inventive rotor, however, adds an annular region extending around the circumference of the rotor that provides an increased mill rotor/mill stator gap between upstream and downstream, relatively smooth, processing surfaces. This region of increased gap is designed to establish a cavitation field to compliment the largely shear-based fluid processing performed by the adjacent smooth rotor surfaces.
In general, according to another aspect, the invention features a colloid mill rotor that comprises a primary processing surface extending annularly around the rotor, and a secondary processing surface, also extending annularly around the rotor downstream of the primary processing surface. An intermediate, annular processing surface is located axially between the primary and secondary processing surfaces and is depressed relative to those surfaces. During operation, the relative operation of the primary and secondary processing surfaces establishes a low pressure region in the enlarged gap created by the intermediate processing surface. This establishes in many cases a cavitation field that compliments the shear processing of the fluid.
In specific embodiments, radially and axially extending slots are provided in the primary processing surface to facilitate the movement of the processed fluid through the gap. These slots in the primary processing surface cooperate with slots in the associated mill stator to facilitate pre-maceration of the fluid.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.